Sitemap.xml is a way for Google to access your website (without being dependant on your internal linking). While it's optional and does not promise indexing, it's especially useful if it's paired with the Google Search Console to monitor indexation.
Including a URL in the sitemap.xml while it is not in your internal linking can allow a page to be indexed, but it's important to note that this URL will not be able to rank in top rankings in Google search result pages.
The main goal is to monitor the indexing of different pages of your site, it will be customary to create a sitemap.xml including all the pages you want to see indexed. This file must be updated automatically according to the lifecycle of the site.
Because your site is large, we recommend the creation and maintenance of a sitemap.xml by
Robots.txt
Sitemap: http://www.cdkeys.com/sitemap-index.xmlSitemap: http://www.cdkeys.com/sitemap-fr.xmlSitemap: http://www.cdkeys.com/sitemap-pt.xmlSitemap: http://www.cdkeys.com/sitemap-it.xmlSitemap: http://www.cdkeys.com/sitemap-en.xmlSitemap: http://www.cdkeys.com/sitemap-es.xmlMore information if needed:
https://support.google.com/webmasters/answer/183668?hl=en
NB
Google Search Console
Provide the sitemap-index.xml in your Google Search Console to track the indexing of your different types of files. Google will automatically retrieve all sitemaps in the index.